Vehicle security systems are widely used to deter vehicle theft, prevent theft of valuables from a vehicle, deter vandalism, and to protect vehicle owners and occupants. A typical automobile security system, for example, includes a central processor or controller connected to a plurality of vehicle sensors. The sensors, for example, may detect opening of the trunk, hood, doors, windows, and also movement of the vehicle or within the vehicle. Ultrasonic and microwave motion detectors, vibration sensors, sound discriminators, differential pressure sensors, and switches may be used as sensors. In addition, radar sensors may be used to monitor the area proximate the vehicle.
The controller typically operates to give an alarm indication in the event of triggering of a vehicle sensor. The alarm indication may typically be a flashing of the lights and/or the sounding of the vehicle horn or a siren. In addition, the vehicle fuel supply and/or ignition power may be selectively disabled based upon an alarm condition.
A vehicle security system may be an aftermarket unit installed after manufacture and initial delivery of the vehicle. A vehicle security system may also be installed as original equipment during the manufacture of the vehicle. In addition, hybrid security systems may also be desirable where a user needs one or more additional functions or features that are not available with an original equipment security system alone.
At least one automobile manufacturer includes a theft-deterring circuit based upon use of designated ignition key having a preselected electrical resistance in a portion thereof. For example, the VATS and newer VATS II systems are used in certain General Motors vehicles to deter vehicle theft. The preselected resistance of the ignition key may typically be one of fifteen values, for example. A resistance sensing circuit portion measures the resistance of the key and will not permit starting and/or running of the vehicle engine unless the preselected resistance is sensed.
Unfortunately, the VATS systems may cause compatibility problems with aftermarket security or remote operating units, such as for remote starting of the vehicle engine. For example, one conventional approach to incorporating a remote start capability in a vehicle including a VATS theft-deterring circuit requires cutting the relatively small gauge wires of the ignition circuit and splicing interconnecting wires thereto. The interconnecting wires are coupled to a switching relay which selectively inserts the predetermined resistance into the ignition circuit thereby permitting remote starting of the vehicle, such as based upon a remote start signal received from a remote hand held transmitter. Remote starting may be desirable in warm or cold climates to allow the air conditioner or heater to operate to achieve a comfortable interior temperature. In addition, a remote start feature may be desirable in certain security related applications to remotely check for sabotage or tampering with the vehicle.
Significant disadvantages are presented by conventional cutting and splicing into the original equipment wiring. For example, considerable time and labor may be required, and the cutting and splicing also increases the chance of errors and may create longer term reliability problems. In addition, a vehicle manufacturer may typically discourage cutting of any original equipment wiring, such as to bypass a VATS system, by disclaiming warranty provisions if original equipment wires are severed.
An example of a remote starting aftermarket unit is disclosed in U.S. Pat. No. 5,184,584 to Cantrell and entitled "Remote Starter for Alarm System Equipped Vehicles". The patent discloses a switching relay and adjustable bypass resistor for interfacing with a VATS-equipped vehicle along the lines described above. The bypass resistance is provided by an adjustable potentiometer that may be difficult to adjust to the desired resistance. Moreover, a potentiometer may drift over time, thereby providing unreliable long term operation.